What took them so long? In other words, why'd they wait until after this became a political "scandal" to sue?

Without getting into the merits at all, this is a pretty obvious answer.

In the space of about a week, you've had a recorded public discussion by an IRS official of the differential treatment of conservative groups (with an apology), the release of an IG report documenting those activities, the initiation of a criminal probe by the Justice Department, public condemnation of the activities by Administration officials up to and including the President, and the departure of the acting head of the agency over these activities. This immediately changes the plaintiffs' likely burden in establishing both the existence and (arguably) culpability of the IRS' conduct from "difficult slog" to "absolute slam dunk."

I doubt that any of these groups could have gotten a lawyer interested in their case based solely on an argument that conservative groups were being differentially treated. Now that differential treatment has been admitted to by the agency, documented, and labelled as wrongful by the Administration, this is an easy case to take to court.